The loopback file system device allows new, virtual file systems to be created, which provide access to existing files using alternate pathnames. Once the virtual file system is created, other file systems can be mounted within it, without affecting the original file system. However, file systems which are subsequently mounted onto the original file system are visible to the virtual file system, unless or until the corresponding mount point in the virtual file system is covered by a file system mounted there.

virtual is the mount point for the virtual file system. dir is the pathname of the existing file system. mflag specifies the mount options; the MS_DATA bit in mflag must be set. If the MS_RDONLY bit in mflag is not set, accesses to the loop back file system are the same as for the underlying file system. Otherwise, all accesses in the loopback file system will be read-only. All other mount(2) options are inherited from the underlying file systems.

A loopback mount of '/' onto /tmp/newroot allows the entire file system hierarchy to appear as if it were duplicated under /tmp/newroot, including any file systems mounted from remote NFS servers. All files would then be accessible either from a pathname relative to '/' or from a pathname relative to /tmp/newroot until such time as a file system is mounted in /tmp/newroot, or any of its subdirectories.

Loopback mounts of '/' can be performed in conjunction with the chroot(2) system call, to provide a complete virtual file system to a process or family of processes.

Recursive traversal of loopback mount points is not allowed. After the loopback mount of /tmp/newroot, the file /tmp/newroot/tmp/newroot does not contain yet another file system hierarchy; rather, it appears just as/tmp/newroot did before the loopback mount was performed (for example, as an empty directory).

All access to entries in lofs mounted file systems map to their underlying file system. If a mount point is made available in multiple locations via lofs and is busy in any of those locations, an attempt to mount a file system at that mount point fails unless the overlay flag is specified. See mount(1M). Examples of a mount point being busy within alofs mount include having a file system mounted on it or it being a processes' current working directory.

Because of the potential for confusing users and applications, you should use loopback mounts with care. A loopback mount entry in /etc/vfstab must be placed after the mount points of both directories it depends on. This is most easily accomplished by making the loopback mount entry the last in /etc/vfstab.